1. Field of the Invention
The present invention relates generally to DC-DC power conversion.
2. Background of the Invention
DC-to-DC power converters are power-processing circuits that convert an unregulated input DC voltage to a regulated DC output voltage. Switched-mode DC-to-DC power converters typically include an inverter, a transformer having a primary winding coupled to the inverter, a rectifying circuit coupled to a secondary winding of the transformer, and a filter connected between the rectifying circuit and the load. The inverter typically includes a pulse width modulated (PWM) switching device (i.e., the primary switch), such as a MOSFET, that converts the DC input voltage to an alternating voltage, which is magnetically coupled from the primary winding of the transformer to the secondary winding. A PWM control circuit regulates conduction of the PWM switching device. The rectifying circuit rectifies the alternating voltage on the secondary winding.
In some applications it is desirable measure the output current of the converter such that the delivered output current can be controlled. For example, if an overcurrent condition exists at the output, the duty cycle of the primary switch may be reduced. In DC-DC converters that are of an isolated type, direct information related to the input current can be easily derived. This is typically done using current sensing resistors or current sensing transformers located in the primary switch current path. Sensors of this type provide information related to the input current or current in the power inductor during the D or energy storage state of the power converter. Since the slope of the output current of the converter (e.g., the current through an inductor of the output filter circuit) is related to the operating point of the converter, attempting to control the output current of a power converter with only the D state information can result in considerable errors.
The use of a resistor as the current sensing element has several advantages, including cost and size. It is therefore desirable to deploy a current sensing resistor as the sensing element. Unfortunately, to manage the power dissipation in the sensing element the developed voltage must be minimized. In an optimized sensor circuit the developed voltage is well below the forward conduction drop of silicon diodes and in general makes direct signal processing difficult and replete with parasitic error terms.
Therefore, there exists a need for a current sensor processing means that develops the average current output of a DC-DC converter using the D state signal information. Additionally, there exists a need for such a processing means that directly interfaces with resistive sensing elements.